Method of downhole drilling and apparatus therefor

ABSTRACT

A downhole drilling apparatus suspended by tubing includes a drill bit driven by an electric motor powered by a cable means. The motor is mounted upon a hollow shaft which allows fluid to pass through, so that a fluid circuit along the tube and back along the annulus between the borehole and the tubing via the drill bit is.established. The state of the motor, particularly its speed and torque, can be monitored, and the motor may then be regulated as a result of the this data. Various sensors may be included in the drilling apparatus, and the data gathered similarly used to regulate the motor. Thrust means are included to urge the drill along the borehole. Supplementary pumps may be provided to assist the fluid flow. Both the thrust means and the pumps may be controlled by the control means.

FIELD OF THE INVENTION

[0001] The invention relates to a method of downhole drilling andapparatus therefor such as an electrically powered bottom hole assemblyfor use in coiled tubing drilling (CTD) applications.

BACKGROUND OF THE INVENTION

[0002] Simple CTD services are known using hydraulic motors to providethe rotational torque in the drill bit using hydraulic pressure of asuitable fluid. Whereas initial efforts at CTD were based aroundremedial work in an existing wellbore, the technology is now used todrill wells from surface and to sidetrack existing wells. Bothoverbalanced and underbalanced drilling techniques have been evaluatedalong with advances in directional drilling technology.

[0003] However there are significant drawbacks with the existinghydraulic motor systems. They have a very low durability, due mainly tothe failure of seals and generally to the problems of transmitting highpressure over long distance in a well. Such failure requires withdrawalof the whole string from the well. Also, conventional coiled tubingdrilling techniques have a limited choice of drilling mediums.

[0004] It is therefore an objective of the present invention to providea method of downhole drilling and apparatus therefor which alleviates orovercomes at least some of these disadvantages.

SUMMARY OF THE NVETION

[0005] According to the invention there is provided an apparatus fordownhole drilling of wells comprising;

[0006] drilling unit comprising a drill bit for penetrating into a rockformation,

[0007] a motor arranged to drive the drill bit, the motor including ahollow shaft which permits the passage of fluid therethrough,

[0008] tubing, upon which the drilling unit and motor are suspended,

[0009] control means which monitor and control the action of the motorand/or drill bit, and

[0010] cable means disposed along the tubing.

[0011] Preferably, the tubing is coiled tubing. Preferably the cablemeans is disposed within the coiled tubing. Preferably the hollow motoris a brushless DC motor providing direct control over the speed andtorque of the drill bit. Preferably at least one sensor is providedbetween the motor and the drill bit. Preferably the sensor or sensorsinclude a rock type sensor such as an x-ray lithography sensor.

[0012] The control means provides the required control over the motor interms of its speed and torque to prevent stalling of the motor and toprovide the most desirable rate of progress of the drilling process. Thecontrol means may be provided with direction output means to control thedirection of the drilling by input to a directional drilling controlmeans. Similarly, the control means may be provided with thrust outputmeans to control the level of thrust of the drilling by input to athruster control means. Preferably the thrust means include a pluralityof eccentric hub type thrusters.

[0013] Also according the present invention there is provided a methodof downhole drilling using an apparatus as defined above.

[0014] Mud may be pumped down the inside of the coiled tubing, throughthe hollow shaft of the motor, and to the bit in order to wash thecuttings away from the bit and back up the well through the annulusformed between the side of the well on the one hand and the outside ofthe coiled tubing and the motor on the other. Or alternatively, mud maybe pumped down the annulus formed between the side of the well on theone hand and the outside of the coiled tubing and the motor on theother, and thence to the bit in order to wash the cuttings away from thebit and back up the well through the hollow shaft of the motor and theinside of the coiled tubing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Various embodiments of the invention will now be described inmore detail, with reference to the accompanying drawings. given as anexample and not intended to be limiting, in which;

[0016]FIG. 1 is a longitudinal elevation of a bottom hole assembly;

[0017]FIG. 2 is a longitudinal section of the bottom hole assembly;

[0018]FIG. 3 is a longitudinal side elevation of a further embodiment ofthe bottom hole assembly;

[0019]FIG. 4 is a schematic general arrangement of a control system ofthe motor of the assembly;

[0020]FIG. 5 is an end elevation of a further embodiment of a motor usedin the assembly:

[0021]FIG. 6 is a side elevation of the further embodiment of the motor.

[0022]FIG. 7 is a schematic general arrangement of a control system ofthe invention:

[0023]FIG. 8 is a further embodiment of the bottom hole assembly in use.

[0024]FIG. 9 is a longitudinal section of the annular pumps

[0025]FIG. 10 is a longitudinal section of the in-line pump

[0026]FIGS. 11. 12 and 13 are cross sections of embodiments the cablemeans showing the annular pumps

[0027]FIG. 14 is a side elevation of a further embodiment of the bottomhole assembly

[0028]FIGS. 15 and 16 shows the thruster and directional actuation meansof FIG. 14 in more detail.

DESCRIPTION OF THE PREFERRED EMBODINT

[0029] Referring to FIG. I and 2, for the first embodiment, anelectrical motor 21 of the type used for electric submersible pumps isused. This electric motor is connected to a planetary gearbox 27 toreduce the output shaft speed to suit the drilling environment.Referring to FIG. 4, the motor is controlled from surface by a laptopcomputer (not here shown) connected to a variable 2o speed drive. Acommand and control software package interrogates the drive to acquireand record real-time drilling data from the motor.

[0030] In this embodiment the system provides enhanced feedback andcontrol of drilling processes in real-time, which, when processedappropriately, will deliver relevant data to the driller and reservoirengineer. The monitoring and control aspects are discussed in moredetail later.

[0031] Referring to an alternative embodiment shown in FIGS. 3, 5 and 6,a modular design is shown which is described in more detail later. Thisembodiment provides a higher specific power output motor 31, and doesnot need a gearbox. Customisable to a wider range of drillingenvironments, this promises to expand the envelope of CT drillingapplications to areas such as hardrock and alternate medium drilling.

[0032] The electric coiled tubing drilling described offers severaldistinct advantages over conventional CTD operations. In particular, thebit speed may be maintained independent of the flow. rate through theCT. The cabling provides a high quality telemetry path for an imunediatedata feedback, and then may be immediately controlled in response to hisdata. The drill bit rotation may easily be reversed, and is morereliable than conventional drilling assemblies. The drilling is suitablefor underbalanced drilling applications and for the dynamic balance ofcirculation and formation pressures.

[0033] The embodiment of the bottom hole assembly illustrated in FIGS. 1and 2 may be split into several distinct components. These are nowdiscussed in more detail.

[0034] The coiled tubing connector 25 provides the electrical andmechanical 25 connections between the power coiled tubing and the bottomhole assembly.

[0035] The connector also directs the flow of drilling fluid around theelectric motor and includes a weakpoint for emergency disconnection. Astandard fishing profile may be included in the design.

[0036] The motor and several parts of the bottom hole assembly must beimmersed in lubricating oil for extended performance. However, duringthe drilling processes and under varying temperature conditions thevolume of this oil will vary. Consequently a simple pressure-balancedcompensation system is incorporated into the design to avoid damage fromoil expansion. This system also provides a quick method of checking theoverall health of the bottom hole assembly prior to running in hole.Checks on fluid levels could give an early indication of oil leakage orseal failure.

[0037] The electric motor 21 used to power the bottom hole assembly is aI5HP electrical submersible pump (ESP) induction motor. A shrouding 26surrounds the motor, allowing the drilling fluid to be pumped throughthe annular space between the shrouding and the motor. This gives thebottom hole assembly outside diameter (OD) of over 130 mm when the OD ofthe electric motor is only 95 mm.

[0038] A specialised industrial gearbox 27 reduces the speed of themotor by a 7:1 ratio. The gear transmission is planetary, and typicallywould be rated to a maximum torque of 290 lbf-ft, though during use themeasured torque may rise above this limit, but the gearbox can withstandthis.

[0039] The gearbox input is connected directly to the motor output shaftvia an adaptor coupling. On the output side, a flex coupling isolatesthe gearbox from the drive shaft. The drive shaft then passes throughtwo sets of bearings and the mechanical seal.

[0040] Below the gearbox, a rotary seal 28 retains the oil in the motorand gearbox whilst the output shaft is rotating. The output shaft speedmakes the use of elastomers unreliable and consequently a mechanicalseal with controlled leakage is used. Typically, the seal is rated foruse up to 10,000 psi differential but designed to slowly leak forlubrication and hence have increased longevity. A bearing pack ofstandard type is connected to the bottom of the drive shaft.

[0041] Referring again to FIG. 4, motor power is supplied by a computercontrolled variable speed drive (VSD). This type of drive is commonlyused to vary the power supply of downhole pumps. A personal computeremulates the internal VSD controller, allowing identical access tocommands and control functions.

[0042] The operator may monitor the bit speed and torque from thecomputer display. Torque is calculated from motor current and bit speedderived from VSD output frequency. A logging system is included tocapture data produced during the testing period to disc. A one minutehistorical sample is also displayed on screen. The control elements ofthe VSD/ laptop are deliberately kept simple to operate by the user. Inthis way, bit speed and or/torque may be quickly altered to suit thedrilling environment and rapidly adapt to changes.

[0043] The drilling fluid is supplied by a portable pumping unit. Fluidenters a 5 swivel on the side of the coiled tubing reel. Somewhat beyondthe swivel connection, a lateral-piece is attached. One side of the T soformed is fed through to the coiled tubing for the fluid path, the otherterminated in a pressure bulkhead, with cable feedthroughs for theelectric cable. Electrical power is supplied by the variable speed drivethrough a set of high power sliprings on the opposite side of the reel.The drilling fluid may be filtered by some conventional method andrecirculated.

[0044] In use, the electric motor drive will try to maintain a constantspeed once set, consequentially there will be a high degree of variationin the torque. As more or less torque is demanded of the motor, thecurrent load will increase or decrease accordingly. As torque isdirectlv related to current, the two fluctuate in unison. The optimumrate of penetration is obtained with a bit speed of between 300-400 rpm.

[0045] As a result of these improvements, the drilling assembly is morereliable. The drilling assembly is more flexible as the bit speed may bemaintained independent of the flow rate, and reversible rotational ofthe bit is possible, of specific interest to traction system and certaincutting operations, such as milling out casing shoes;

[0046] Since there is immediate data feedback via a high quality, highdata rate telemetry path providing information to the drilling engineerfor geosteering and other applications. With the data from the drillingprocess; torque at bit, bit condition, performance drop-off evaluationfor optimal ROP may all be determined

[0047] The drilling assembly is suitable for a wider range of drillingtechnologies such as underbalanced, hard rock and alternate mediumdrilling, and temperatures, drilling applications, and aggressivedrilling media

[0048] The system incorporates the power and telemetry infrastructureupon which numerous other applications can piggy-back, providing amodular bottom hole assembly which is customisable to a wider range ofdrilling applications and environments. Ideally, integrated sensors areincluded in is the bottom hole assembly to provide the real-time datarequired to make timely and informed drilling decisions. The data fromthe sensors may be transmitted by a cable parallel to the power cable,or the data may be superimposed upon the power line itself.

[0049] The system also offers certain advantages in terms of coil life.Primarily, fatigue is reduced as hydraulic energy is no longer requiredto drive the PDM. Secondly, stall-out situations can be avoidedelectronically, reducing the need to cycle the CT up and down each timethe PDM assembly stalls.

[0050] The bottom hole assembly may be wired into surface sensors fromthe coiled tubing unit to be sensitive to changes in weight on bit andROP. Feedback and control loops can be added to keep constant ROP orconstant weight on bit whilst varying the other available drillingparameters. Downhole tools may also be added for geologicaldetermination. It is also possible to enable integration of downholedirectional sensors and geosteering capability. Thus a fully automateddrilling system will be able to follow a predetermined course to locategeological targets with minimal correctional changes in direction. Thiswould be designed to reduce doglegs and their associated problems. Sucha drilling system could also be programmed to optimise ROP.

[0051] Referring to FIGS. 3, 5 and 6, the motor 31 includes rotorelements 38, stator elements 39 and a hollow shaft 34 which permits thepassage therethrough of fluid from the inside of the coiled tubing tothe drill bit 32. Mud is pumped from the surface down the inside of thecoiled tubing 33 through the bore 35 of the hollow shaft 34 shaft and tothe bit 32 to wash the cuttings away from the bit and back along thewell being cut on the outside of the motor and continuous coiled tubing.A liner tube 37 running through the hollow shaft ensures that the motorcomponents are kept free of contamination, and that the need for sealswithin the motor is reduced.

[0052] The hollow motor is a brushless DC motor which provides directcontrol over the speed and torque of the drill bit 32. The rotors 38 andstators 39 of the motor are disposed in segmented sections along thehollow shaft 34, each section being separated from the next by bearings40 supporting the hollow shaft. This arrangement allows the motor toadopt a greater curvature without the moving parts of the motor beingforced to touch and damaging the motor and reducing its efficiency,since the regions between the motor sections are able to curve to agreater degree.

[0053] A sensor support 37 is provided between the motor 31 and thedrill bit 32. The sensor support 37 is provided with a rock tvpe sensorsuch as an x-ray lithography sensor as well as pressure and temperaturesensors.

[0054] As shown in FIG. 4 control means 41 comprising a digitalestimator and a motor simulator are provided for controlling the motor31. Voltage and current input means 42 are provided to determine thespeed and torque of the drill bit to the control means 41 which arepreferably provided by direct electrical measurements of the motor.Preferably formation type input means are also provided to the controlmeans from the rock type sensor 37. Also drill bit type input means areprovided to input the type of drill bit being used corresponding to theparticular drilling operation. Thus power and data is provided to themotor by means of the cable 43.

[0055] The control means provides the required control over the motor interms of its speed and torque to prevent stalling of the motor and toprovide the most desirable rate of progress of the drilling process.

[0056]FIG. 7 shows the possible interaction between some of thedifferent components. The electric motor is directly controlled by abottom hole computer via link 69, as well as being influenced by thedownhole sensors by link 67 (which could also be fed firstly to thebottom hole computer). The bottom hole computer, and some of thedownhole sensors, also monitor the motor's performance, that is, thedata transfer is bidirectional.

[0057] The surface computer gathers data from the bottom hole computertransmitted along the cable 38 a, and also directly from the downholesensors along cable 38 b, and also sends the drill operator's commandsthe bottom hole computer when the drilling is to be altered. Inlinetools, such as the steering means, a traction tool and its load cell, asupplementary pump, and a flow tester are also included in the bottomhole assembly, with bidirectional communication between both the surfaceand bottom hole computers by cable 38 a, and in the case of the tractiontool and its load cell, between each other. Naturally, many differentarrangements are possible, a particular arrangement being dependent,among other things, on the particular cable means and tools employed.

[0058]FIGS. 8 and 9 show a further embodiment of the bottom holeassembly with a thruster 50 and knuckle joint 52 provided on the bottomhole assembly. FIG. 8 shows the thruster and knuckle joint beingactivated, the thruster urging the drill along the borehole, and theknuckle joint causing the direction of the drill to be changed. FIG. 9shows the thruster and knuckle joint being de-activated. The controlmeans is provided with direction output means to control the steering ofthe drilling by providing the required input instructions to the knucklejoint 52. Similarly, the control means is provided with thrust outputmeans to control the level of thrust of drilling by input to thethruster 50. The thrusters may be of the active variety, such as theeccentric hub type thrusters shown here, or thrust may be passivelyprovided, by applying more force to the tubing at the mouth of theborehole, or a combination these means may be used by the control meansto apply more weight to the bit and urge it forward, maintaining themost effective penetration rates whilst at the same time preventingstalling of the motor or failure for other reasons. Also the controlmeans provides control over the direction of the drilling bit whichenables the tool to automatically drill in the required direction, whichmay be changed to avoid certain rock formations or changed in responseto other information of the formation which has been received duringdrilling. Other types of machinery or downhole tools may be includedwith the bottom hole assembly and similarly controlled by the controlmeans.

[0059]FIG. 11 shows a general arrangement of the components of theapparatus of a further embodiment showing multiple thruster means 54which are provided to enable the horizontal drilling over longdistances. This is used for example for the drilling out to sea from aland based drilling platform to avoid the expense of an off shoreplatform. Similarly horizontal drilling is useful from a sea basedplatform to reduce the need to erect additional sea based platforms. Themultiple thrusters can all be controlled by the same control means sothat the drilling operation can be effectively controlled along thewhole length of the coiled tubing and existing problems of failure ofmotors and other components can be avoided and permit much longer wellsto be drilled.

[0060]FIG. 11 also shows supplementary pumps 60 disposed along thecoiled tubing 23 to assist the fluid flow in the well. These pumps maybe disposed so as to act in the annulus between the outer diameter ifthe coiled tubing and the well, or in the coiled tubing. The fluid maybe caused to flow into the well through the coiled tubing and thence outof the well by the annulus, or in the opposite direction, that is, intothe well through the annulus and out through the coiled tubing.

[0061] The pumps to be disposed so as to act in the annulus are hollowbored so that the coiled tubing may pass through the pumps. Referring toFIG. 12, the annulus pump has a hollow shaft with a motor and set ofturbine blades 62 set upon it, the coiled tubing 23 passing through theshaft. The power connections 64 to the pump's motor are similar to thoseof the hollow motor driving the drill bit, that is, they are of thebrushless DC type. Arrows are shown to indicate the possible flowpattern of fluid. Naturally, one may choose to cause the fluid to flowdown the coiled tubing and to return up the annulus, or vice versa. Thepump may be secured within the borehole 70 by securement means 72.

[0062] FIGS. 14 to 16 also show various arrangements of the cable means43 disposed within the coiled tubing 23, preserving a sufficient bore 35through the coiled tubing for fluid flow. As shown in FIG. 14, the cable43 may be of the coaxial type concentric with the coiled tubing, or, asshown in FIGS. 15 an 16, a three strand type, either disposed in anannular steel setting 44, or set within a cable 45 running within thecoiled tubing. The cable means could even be strapped to the outside ofthe coiled tubing.

[0063] Referring to FIG. 13, the pumps 66 fitted in-line with the coiledtubing and acting on the flow within the coiled tubing 23 includeturbine blades mounted conventionally upon a solid shaft 68, the shaftbeing caused to turn in order to turn the blades.

[0064] Although the principles disclosed here are eminently suited fordrilling with coiled tubing, they are not so limited. Referring to FIG.17, the techniques described above may be applied to jointed drill pipe.A drill string 80 composed of jointed sections of drill pipe terminateswith a drill bit. Disposed within the drill string is a cable 82supplying power to an electric motor 84 which drives the drill bit 22.Sensors are also included at the end of the drill string, data gatheredfrom these being transmitted using the power cable 82. The cable isattached to the motor by a stab-in connector 86, so that the cable maybe disconnected to allow further pipe sections to be added to the drillstring. Fluid is then pumped down the borehole annulus to return up thedrill string or vice versa, whilst the drill bit is electricallyoperated, being regulated by the control means in response to therelevant data collected. FIG. 18 shows the bottom hole assembly beingdeployed from a vessel 90. Fluid is pumped down a supply line 20 to afluid accumulator 92 located upon the well head 94. The fluid is thenpressurised and passes into the pressure lock chamber 96 and flows downinto the borehole 70, in the annulus formed around the coiled tubing 23.The fluid passes into the drill bit 22 and thence up through the coiledtubing and back to the vessel for filtering and recirculating. Thepressure lock chamber included dynamic seals 98 which allow the coiledtubing to be fed into the borehole whilst the pressure is maintained.Pump, motor and traction units 100 aid the fluid flow as well asaltering the weight on bit.

[0065] Alternative embodiments using the principles disclosed willsuggest themselves to those skilled in the art, and it is intended thatsuch alternatives are included within the scope of the invention, thescope of the invention being limited only by the claims:

What is claimed is:
 1. An apparatus for downhole drilling of wellscomprising; a drilling unit comprising a drill bit for penetrating intoa rock formation, a motor arranged to drive the drill bit, the motorincluding a hollow shaft which permits the passage of fluidtherethrough, tubing upon which the drilling unit and motor aresuspended, control means which monitor and control the action of themotor and/or drill bit, and cable means disposed along the tubing.
 2. Anapparatus according to claim 1, wherein the tubing is a length ofcontinuous coiled tubing.
 3. An apparatus according to claim 1, whereinthe cable means is disposed within the tubing.
 4. An apparatus accordingto claim 1, wherein the hollow motor is a brushless DC motor providingdirect control over the speed and torque of the drill bit.
 5. Anapparatus according to claim 1, wherein at least one sensor is providedbetween the motor and the drill bit.
 6. An apparatus according to claim5, wherein the sensor or sensods include a rock type sensor such as anx-ray lithography sensor.
 7. An apparatus according to claim 6, whereinformation type input means are provided to the control means from therock type sensor.
 8. An apparatus according to claim 5, wherein the atleast one sensor includes pressure and temperature sensors.
 9. Anapparatus according to claim 1, wherein speed and torque input means areprovided to the control means.
 10. An apparatus according to claim 9,wherein the speed and torque input means are provided by directelectrical measurements of the motor.
 11. An apparatus according toclaim 5, wherein drill bit type input means are provided to input thetype of drill bit being used corresponding to the particular drillingoperation.
 12. An apparatus according to claim 1, wherein the controlmeans is provided with direction output means to control the directionof the drilling by input to a directional drilling control means.
 13. Anapparatus according to claim 1, wherein the control means operates athrust means capable of urging the drill along the well.
 14. Anapparatus according to claim 13, wherein the thrust means includes aneccentric hub type thruster.
 15. An apparatus according to claim 13,wherein the thrust means is a plurality of thrusters arranged along thelength of the coiled tubing to enable the drilling of very long wells.16. A method of downhole drilling using an apparatus according toclaim
 1. 17. A method according to claim 16, wherein mud is pumped downthe inside of the coiled tubing, through the hollow shaft of the motor,and to the bit in order to wash the cuttings away from the bit and backup the well through the annulus formed between the side of the well onthe one hand and the outside of the coiled tubing and the motor on theother.
 18. A method according to claim 16, wherein mud is pumped downthe annulus formed between the side of the well on the one hand and theoutside of the coiled tubing and the motor on the other, and thence tothe bit in order to wash the cuttings away from the bit and back up thewell through the hollow shaft of the motor and the inside of the coiledtubing.
 19. A pump disposed along the tubing which causes or supplementsthe method according to claim 16.`